This invention relates generally to force sensors, such as pressure sensors and accelerometers by way of example, and more specifically it relates to a method for fabrication of thick film resistor force sensing elements used in such sensors.
A paper entitled, "Integrated Hybrid Pressure Sensor Using A Piezoresistive Thickfilm Sensor Element", SAE International Congress 1987, Paper 870286, by H.J. Glas, W. Kuhnt, G. Stacher, discloses certain methodology relating to the fabrication of a pressure sensing element for use in a pressure sensor. A copy of that paper is attached to the accompanying Information Disclosure Statement.
The present invention represents a methodology which possesses significant cost and performance advantages over prior technology for the fabrication of piezoresistive thick film force sensing elements. One of the unique features of the methodology of the present invention relates to the use of unfired ceramic tape as one of the components from which a force sensing element of this particular type is fabricated. Such tape is a conventional commercial product, an example of which is Dupont Electronics "Green Tape".TM. system. A copy of descriptive material from Dupont Electronics is also attached to the accompanying Information Disclosure Statement. A generic definition of "Green Tape" is a sheet of unfired ceramic material. The material is flexible in this state. The tape can be stamped in the unfired condition and then fired at a high temperature to finish the product. The tape is fabricated by tape casting a slurry of ceramic and binders. This process is used for fabricating hybrid thick film substrates and ceramic multi-layer capacitors.
Also enclosed as a part of the Information Disclosure Statement is a paper entitled, "Low Temperature Co-fireable Ceramics: A New Approach For Electronic Packaging", 36th Electronics Component Conference (1986), Proceedings, pp 37-47 by A.L. Eustice, S.J. Stewart, A.R. Travis, and H.T. Sawhill. Another paper, "Low Temperature Co-fireable Ceramics With Co-fired Resistors" by H.T. Sawhill, A.L. Eustice, S.J. Horowitz, J. Gar-El and A.R. Travis is also attached to the Information Disclosure Statement.
As one can see from consideration of the state of the art represented by the documentation included with the Information Disclosure Statement, no-one has heretofore recognized the possibility of using unfired ceramic tape in the fabrication of piezoresistive thick film force sensing elements for pressure sensors, accelerometers, or other force sensors, in such a manner that in the completed sensing element, a portion of the fired tape forms a deflectable force sensing zone, and wherein a conductor and thick film resistor system printed and fired onto the ceramic tape is disposed in relation to the force sensing zone such that a characteristic of the system provides a signal which correlates with the force applied to the force sensing zone.
The fabricated sensing element is adapted to be a principal component part of a force sensor. The typical force sensor comprises a suitable construction for the mechanical mounting of the force sensing element in such a manner that the force sensing zone thereof is properly disposed to sense the force of interest, and also for the electrical connection of the conductor and resistor system with electrical terminal structure of the sensor such that the sensor can be connected in an electric circuit to provide an electric signal representation of force applied to the force sensing zone of the sensing element.
Principles of the invention will be disclosed in the ensuing detailed description of a presently preferred embodiment in accordance with the best mode contemplated at the present time for carrying out the invention. Drawings (not necessarily to scale or relative proportions) accompany the disclosure and are identified as follows: